RF measurements during floating MD in Week 40 3 rd of October 2012 LIU-SPS BD WG 25/10/2012 Participants: T. Argyropoulos, H. Bartosik, T. Bohl, J. Esteban Müller, H. Timko, E. Shaposhnikova CCC: G. Iadarola, Y. Papaphilippou, G. Rumolo Thanks to all SPS OP on shift 1

2 General MD title: Longitudinal set up of the 25 ns LHC beam with Q20 optics (nominal ~1.25x10 11 pb and high ~1.4x10 11 p/b injected intensities) MD aim:  Improve the bunch length distribution inside the batch (“U-shape”)  Improve the slope of the bunch lengths along the train  Achieve beam stability and acceptable beam parameters at flat top Beam conditions:  4 batches of 72 bunches  Intensities at injection: N p ~1.25x10 11 p/b and N p ~1.45x10 11 p/b Varying  RF voltage amplitudes at FB (V 200 and V operation always in double RF)  Controlled longitudinal emittance blow-up (Amplitude and scaling – always on )  Phase between the 2 RF systems at FT (first attempt to improve stability by compensating for beam loading at 800 MHz)

Outline  Losses  Nominal intensities (~1.25x10 11 at injection)  Optimization of the controlled longitudinal emittance BUP to reduce the “U-shape” of the bunch length distribution inside the batches at FT  Modification of the RF voltage (V 200 ) to improve the slope of the bunch lengths along the batch trains at FT  High intensities (~1.45x10 11 at injection)  Improve stability: mainly by optimizing the BUP (amplitude, scale)  Summary 3

4 Losses  Very good transmission for nominal intensities (~96 %) : not affected by the changes in longitudinal parameters  Still low losses for the higher intensities (~6-7 %)

5 Nominal intensities (~1.25x10 11 at injection)  Beam was stable (with the 800 MHz RF system and controlled emittance blow-up)  Decrease of average bunch length along the FB for constant RF voltage (3 or 4.5 MV) + slight increase (~1%) of capture losses  not observed with the voltage dips  Bunch length distribution at FT:  “U-shape” inside the batch  slope along the batch train Example of the best conditions:  V 200 = 4.5 MV  V 800 = 0.45 MV  Scale BUP = 0.93  V BUP = 35 mV

6 U – shape of bunch length distribution  Due to beam loading the synchrotron frequency distribution varies along the batch  Different effect of the controlled BUP to the bunches at the edges and the center of the batch  Leads to the “U-shape” at FT  Reduce this by decreasing the scale parameter in the BUP Synchrotron frequency Distribution inside the bunch. Before BUP – V 800 /V 200 = 0.1  Average of all acquisitions for different BUP scales: V 200 = 4.5 MV – V 800 = 0.1V 200 – V BUP = 30 mV increasing the scale makes bunch lengths along the batch more uniform  with scale=0.85 bunches a slightly unstable at FT  slightly better stability for scale=0.9 than 0.93 (mainly dipole oscillations)

7 Slope along the batch train  Correlation of the V 200 at FB with the bunch length slope along the batch at FT  V 800 = 0.45 MV – V BUP = 35 mV – Scale BU = 0.93  Not significant improvement with the different voltage settings that were tried  Small difference in stability

8 Higher intensities (~1.45x10 11 at injection)  Beam was unstable - with the 800 MHz RF system and controlled emittance blow-up (same settings as in the low intensities)  Decrease of average bunch length along the FB for constant RF voltage  Bunch length distribution at FT: “U-shape” inside the batch and slope along the batch train still remain  Optimize the controlled emittance BUP:  increase the noise amplitude V BUP  lower the scale parameter  Limiting time with these intensities  only few acquisitions

9 Higher intensities (~1.45x10 11 at injection)  Increase V BUP while keeping the same high scale (0.9) didn’t show any improvement V 200 = 3 MV – V 800 = 0.45 MV Scale BUP = 0.9 – V BUP = 35 mV V 200 = 3 MV – V 800 = 0.45 MV Scale BUP = 0.9 – V BUP = 60 mV

10 Higher intensities (~1.45x10 11 at injection)  Increase Scale BUP while keeping the same high V BUP (60 mV) improved stability  Still unstable with long bunches at FT V 200 = 3 MV – V 800 = 0.3 MV Scale BUP = 0.85 – V BUP = 60 mV V 200 = 3 MV – V 800 = 0.45 MV Scale BUP = 0.9 – V BUP = 60 mV Change of the ratio V 800 /V 200 to 0.1 improved the situation but not significantly

11 Higher intensities (~1.45x10 11 at injection)  Increase RF voltage at FB improved stability  Still some bunches are unstable with very long bunches at FT V 200 = 4.5 MV – V 800 = 0.45 MV Scale BUP = 0.85 – V BUP = 60 mV V 200 = 3 MV – V 800 = 0.3 MV Scale BUP = 0.85 – V BUP = 60 mV

12 Summary  Good transmission:  ~96-97 % for Np~1.25x10 11 p/b  ~93-94 % for Np~1.45x10 11 p/b  Stable beam always with the nominal intensities:  800 MHz ON  controlled longitudinal emittance BUP ON  Decrease of average bunch length along the FB for constant RF voltage (3 or 4.5 MV)  not observed with the voltage dips  but slightly more unstable at FT  “U-shape” inside the batch :  improved with higher Scale BUP  slope along the batch train remains still  further investigation is needed  Higher intensities  Beam unstable at FT  small improvement by optimizing the BUP  but bunches too long at FT  More time is needed